This chapter reviews two diseases such as chlamydial abortion and chlamydial endometritis, which are likely to be of interest to workers in the field of human chlamydiosis. The causal agent of enzootic abortion of ewes (EAE) is the ovine abortifacient form of Chlamydia psittaci, previously designated biotype 1 and immunotype 1. Most studies have reported a remarkable degree of homogeneity of strains of C. psittaci in molecular terms. When EAE is experimentally reproduced in naive ewes by subcutaneous injection or by dosing via the oropharyngeal route, abortions usually occur in 40 to 50% of animals, with 75 to 90% excreting chlamydiae at lambing or at abortion. A monoclonal antibody against another surface-expressed antigen of C. psittaci, an 89-kDa protein, reduced infection by 60% in a tissue culture model, suggesting that antibodies against this protein may also be involved in acquired immunity. Three principal antigenic preparations have been used in vaccines: (i) inactivated forms of the cultured organism; (ii) a temperature-sensitive, attenuated mutant; and (iii) protein extracted from genetically engineered bacteria expressing the major outer membrane protein. Due to the relatively recent recognition of bovine chlamydial infertility, metritis, and salpingitis, the most reliable descriptions of its clinical expression are provided by experimental infection studies. Both forms of chlamydial infection of ruminants offer useful comparative features for human reproductive tract disease. The bovine disease is still in the very early stages of characterization, and so the full extent of its usefulness as a model for human disease remains to be determined.